1. Field of the Invention
This invention relates to cast, cobalt-base alloys and, in particular, relates to cobalt-base alloys that are particularly useful for high temperature service under corrosive conditions.
2. Brief Statement of the Prior Art
Cobalt-base alloys have been developed with various alloying elements to achieve prolonged life in high temperature and corrosive gaseous environments. This development has been primarily directed to providing suitable metallurgy for gas turbine engines and, in particular, for turbine stator vanes which are contacted by hot combustion gasses. The general objective for such applications is to furnish a metal having an extended service life under prolonged stress, stress cycling and corrosive attack as experienced in gas turbine service. For such applications, the metal should have sufficient initial ductility to withstand the hardening or embrittlement that frequently accompanies its use under these service conditions. Additionally, the metal should have a high tensile strength and creep resistance through a wide temperature range of expected applications, e.g., from about 1000.degree.F up to or approaching combustion gas temperature such as up to about 2000.degree.F. Resistance to corrosive agents such as sulfides and chlorides encountered in gases is also required.
Cobalt-base alloys and, in particular, carbide-strengthened, cobalt-base alloys have been developed for this service and have exhibited excellent service life. Generally, these alloys contain a matrix formed principally of cobalt with chromium, tungsten and nickel as matrix alloying elements and with carbides of tantalum, zirconium and titanium. Typical of such alloys is that described in U.S. Pat. No. 3,432,294.
Zirconium is not entirely suitable for use in the alloy because it reacts both with crucible used for melting, as well as the ceramic mold materials encountered during casting of the machine elements, frequently resulting in rejection of the molded parts. This is reported at A.S.M. Metals Engineering Quarterly, Vol. 9, No. 2, pp 24-45, May, 1969, "Casting Cobalt-Base Superalloys", by M. J. Woulds. The mold reactivity is particularly acute in large section thicknesses of castings, where the metal in contact with the shell mold remains hot for prolonged periods, allowing the metal-mold reaction to proliferate. Accordingly, it is desirable to provide a zirconium-free, cobalt-base alloy.